The present invention relates to a process for converting hydrocarbons. More particularly, the invention relates to a catalytic hydrocarbon conversion process in which oxidation reactions occurring during catalyst regeneration are enhanced.
Catalytic cracking systems employ catalyst in a moving bed or a fluidized bed. Catalytic cracking is carried out in the absence of externally supplied molecular hydrogen, and is, therefore, distinctly different from hydrocracking, in which molecular hydrogen is added in processing. In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, hydrocarbon feed is contacted with catalyst particles in a hydrocarbon cracking zone, or reactor, at a temperature of about 425.degree. to 600.degree. C., usually 460.degree. to 560.degree. C. The reactions of hydrocarbons at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are separated from the cokedeactivated, spent catalyst and are withdrawn from the reactor. The coked catalyst particles are stripped of volatiles, usually by means of steam, and passed to the catalyst regeneration zone. In the catalyst regenerator the coked catalyst is contacted with a predetermined amount of molecular oxygen. A desired portion of the coke is burned off the catalyst, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 540.degree. to 815.degree. C., usually 590.degree. to 730.degree. C. Flue gas formed by combustion of coke in the catalyst regenerator may be treated for removal of particulates and conversion of carbon monoxide, after which it is normally discharged into the atmosphere.
The extent of conversion obtained in a cracking operation may be defined as the volume percent of feed hydrocarbons having a normal boiling point of at least 221.degree. C. changed to hydrocarbon products having normal boiling points below 221.degree. C. during the conversion step. Conversion is often used as a measure of the severity of a commercial cracking operation. At a given set of operating conditions, a more active catalyst gives a greater conversion than does a less active catalyst. High conversion allows flexible operation of an FCC unit. For example, when conversion is raised, feed throughput can be increased, or a higher degree of feed conversion can be maintained with a constant throughput rate.
The selectivity with which the feed is converted to desired hydrocarbon products is also important. The primary desired product is naphtha-boiling-range hydrocarbons, i.e., products which have a normal boiling point roughly between 24.degree. C. and 220.degree. C. Other possible conversion products are usually not as valuable as naphtha. For example, feed conversion may yield coke, hydrogen and such normally uneconomical by-products as the light paraffins, methane, ethane and propane. Formation of some coke is needed to provide process heat, but excessive coke formation at the expense of naphtha is undesirable. In contrast to light paraffins, the light olefins may be economically attractive products. For example, C.sub.2 -C.sub.4 olefins can be utilized as feeds for other hydrocarbon conversion processes, such as alkylation of isobutane to form high-octane gasoline components, or for petrochemical-type operations such as polymerization.
The relatively recent impetus for eliminating octane-improving additives such as lead and manganese from commercial gasoline has increased the importance of improving the clear octane of the various components which make up the gasoline pool. Catalytically cracked naphtha is an important source of gasoline in the United States, but is often among the lower-octane components employed in the unleaded gasoline pool. FCC-derived naphtha may be upgraded by any of several conventional refining operations such as reforming, but the added expense of further refining can advantageously be avoided if octane ratings of FCC naphtha can be raised to an acceptable level by modifying the catalytic cracking operation itself.
Several patents have suggested modification of cracking catalysts to increase the octane rating of FCC naphtha. The modifications usually involve adding to the catalyst particles a metal or metal compound having a moderate dehydrogenation activity. For example, U.S. Pat. No. 3,835,032 suggests the use of silver-exchanged or copper-exchanged Type Y zeolite component in a cracking catalyst to increase the aromatic content of the naphtha product. U.S. Pat. No. 3,929,621 suggests the use of copper-exchanged Y zeolite as a catalyst component in order to increase aromatics and olefins in the cracked naphtha. U.S. Pat. No. 3,994,800 suggests using a modified Type Y zeolite to raise the olefins content of catalytically cracked naphtha. U.S. Pat. No. 3,788,977 suggests addition of alumina-supported uranium or platinum either as a component of cracking catalyst particles or in a particulate additive introduced into circulation in the particulate solids inventory in a cracking system to increase the dehydrogenation and cyclization reactions taking place in the cracking reactor.
Metals and metal compounds which have a substantial dehydrogenation activity generally also have a substantial activity for oxidation catalysis. The desire to promote efficient combustion of carbon monoxide during regeneration of cracking catalyst has led to the use of highly active metals as components in cracking catalysts. Some commercial cracking systems regenerate catalyst in an incomplete carbon monoxide combustion mode, in which a substantial amount of carbon typically remains on the catalyst after regeneration, e.g., more than 0.2 weight percent, usually about 0.25 to 0.45 weight percent residual carbon. The flue gas removed from cracking catalyst regenerators operating in an incomplete combustion mode is characterized by relatively low carbon dioxide/carbon monoxide volume ratio. The amount of oxygen introduced into a catalyst regenerator operating in an incomplete combustion mode must usually be carefully limited in order to prevent afterburning, or combustion of carbon monoxide in the flue gas downstream of the dense bed of catalyst, with consequent overheating of the flue gas.
Most fluidized catalytic cracking systems now use zeolite-containing catalysts. The zeolite catalysts have high activity and selectivity, particularly when the concentration of coke on the regenerated catalyst is relatively low. It is therefore desirable to have as little coke as possible on regenerated zeolite-containing catalysts to obtain the high activity and selectivity. It is often desirable to burn carbon monoxide as completely as possible during catalyst regeneration to obtain additional heat, especially when the concentration of coke on spent catalyst is low as a result of high catalyst selectivity. Among the ways previously suggested for reducing the amount of coke on regenerated catalyst and burning carbon monoxide for process heat is catalysis of complete combustion of carbon monoxide in a dense-phase catalyst bed in the regenerator by an active, combustion-promoting metal. For example, platinum has been used either as an integral component of the cracking catalyst particles or as a component of a separate particulate additive mixed with catalyst particles in the circulating particulate solids inventory. Various ways of employing carbon monoxide combustion-promoting metals in cracking systems have been suggested. In U.S. Pat. No. 2,647,860, it is proposed to add 0.1-1 weight percent chromic oxide to a cracking catalyst to promote combustion of carbon monoxide to carbon dioxide and thereby to prevent afterburning. In U.S. Pat. No. 3,808,121, it is proposed to introduce relatively large-size particles containing a carbon monoxide combustion-promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, comprised of relatively small-size catalyst particles, is cycled between the cracking reactor and the catalyst regenerator, while the combustion-promoting particles remain in the regenerator because of their size. Oxidation-promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., disposed on an inorganic oxide such as alumina are disclosed. Belgian Patent Publication No. 820,181 suggests using catalyst particles containing platinum, palladium, iridium, rhodium, osmium, ruthenium or rhenium to promote carbon monoxide oxidation in a catalyst regenerator. The publication suggests addition of an amount of the metal between a trace and 100 ppm to the catalyst particles, either during catalyst manufacture or during the cracking operation, as by addition of a compound of the combustion-promoting metal to the hydrocarbon feed. The publication also teaches that addition of the promoter metal to the cracking system lowers product selectivity by substantially increasing coke and hydrogen formation. Catalyst particles containing the promoter metal can be used alone or circulated in physical mixture with catalyst particles free of the combustion-promoting metal. U.S. Pat. Nos. 4,072,600 and 4,093,535 discloses the use of combustion-promoting noble metals in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
It is recognized in the cracking art that metals which actively catalyze dehydrogenation and oxidation reactions can have serious drawbacks when used in cracking catalysts. The presence of such metals in a cracking catalyst can enhance formation of coke, hydrogen and light paraffin gases such as methane, as observed, for example, when processing hydrocarbon feeds which are high in nickel. The art has suggested several ways to obtain oxidation catalysis benefits by using active metals in a cracking catalyst without suffering a loss in product selectivity. In one approach, hydrocarbons are prevented from contacting the catalytic metal. Only the oxidation activity of the metal is utilized, since the metal is retained in the catalyst regenerator (e.g., U.S. Pat. No. 3,808,121) or is included within the crystals of a zeolitic crystalline aluminosilicate which has pore openings no larger than 5 Angstroms and small size (3-5 Angstrom) cages, so that essentially no feed or cracked hydrocarbons can contact the metal while it is in the reactor (e.g., U.S. Pat. No. 3,364,136). In another approach, the catalytic metal is employed on the catalyst at a concentration sufficiently low that the adverse effect on selectivity is small enough so that the selectivity loss caused by the presence of the metal can be economically offset by the benefits of increased heat due to complete carbon monoxide combustion and increased zeolite activity and selectivity due to low residual coke level.
U.S. Pat. No. 3,267,043 describes the preparation of a chromia-boria xerogel by reacting a chromium compound, boric acid and an epoxide in the presence of a solvent capable of dissolving the chromium compound to form a hydrogel, and dehydrating the hydrogel. The composition is suggested for use as a catalyst for dehydrogenation and isomerization of paraffins.
U.S. Pat. No. 4,071,436 describes the use of alumina particles containing 0.07 to 3.4 weight percent chromium to reduce the level of sulfur oxides in catalyst regenerator flue gas in a catalytic cracking system. The use of a low concentration (1 weight percent or less) of chromium in catalyst particles in order to enhance carbon monoxide combustion has been suggested in U.S. Pat. No. 2,647,860. This patent teaches that use of more than 1 weight percent chromium causes excessive coke formation selectivity loss. U.S. Pat. No. 4,093,535 shows results obtained using catalysts impregnated with 0.05 weight percent chromium, 0.2 weight percent chromium and 1 weight percent chromium in cracking and oxidation activity tests. This patent discloses that use of catalyst containing 1 weight percent chromium caused a severe loss in cracking activity and some loss in product selectivity while providing only a small increase in oxidation activity (CO.sub.2 /CO ratio of 3.8) relative to catalyst without an oxidation promoter (CO.sub.2 /CO ratio of 1.7). The prior art teachings as to chromium use indicate that chromium is generally believed to be only slightly effective for enhancing oxidation reactions during catalyst regeneration, and that use of catalyst containing 1 weight percent or more chromium results in severe loss of activity and selectivity in cracking.
Three U.S. Pat., Nos. 4,146,463, 4,153,534, and 4,153,535, allege that chromium can be used in low concentrations (10 ppm to 1 weight percent) in catalytic cracking systems to adsorb sulfur oxides in a catalyst regenerator, in order to reduce sulfur oxides emissions in catalyst regenerator flue gas.